The Direct Detection of Cool, Metal-Enriched Gas Accretion onto Galaxies at z 0.5

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We report on the discovery of cool gas inflow toward six star-forming galaxies with redshifts z 0.35 - 1. Analysis of MgII and FeII resonance-line absorption in Keck/LRIS spectroscopy of this sample reveals velocity shifts of 80 - 200 km/s and equivalent widths for inflowing gas of > 0.6 Ang in five of the six objects. The host galaxies exhibit a wide range in star formation rates (SFR 1 - 40 Mₛun/yr) and have stellar masses similar to that of the Milky Way (log M*/Mₛun 9.6 - 10.5). Imaging from the Hubble Space Telescope Advanced Camera for Surveys indicates that five of the six galaxies have highly inclined (i > 55 deg), disk-like morphologies. These data represent the first unambiguous detection of inflow into isolated, star-forming galaxies in the distant universe. We suggest that the inflow is due to the infall of enriched material from dwarf satellites and/or a galactic fountain within the galaxies. Assuming that the material has been enriched to 0.1Zₛun and has a physical extent approximately equal to that of the galaxies, we infer mass inflow rates of dMᵢn/dt > 0.2 - 3 Mₛun/yr for four of these systems. Finally, from comparison of these absorption lines to the profiles of MgII and FeII absorption in a larger spectroscopic sample of 100 objects, we measure a covering fraction of cool inflow of at least 6%, but cannot rule out the presence of enriched infall onto as many as 40 of these galaxies.